This invention relates to a method of press-forming corrugated paperboard into an at least partly curved board which serves as the substrate of a wall covering material for the interior of cars, ships or buildings. The invention is particularly concerned with the prevention of cracking in relatively deep-drawn areas of the press-formed board.
In the field of wall covering board materials to afford comforts and/or decorative effect to the interior of cars, ships or buildings, corrugated paperboard has recently attracted increasing attention as a basic material because of its light weight, low cost, good heat and sound insulating ability and fairly good adaptability to various mechanical processing. In automobiles, for example, trim boards such as roof trim and door trim boards of corrugated paperboard base have already been in practical use on an industrial scale.
Trim boards of this type usually consist of a substrate of corrugated paperboard and a skin or facing layer which is laid on one side of the substrate for producing protective, decorative and/or cushioning effects. In most cases the corrugated paperboard substrate takes the form of a double-faced board given by bonding liners to both sides of a corrugated medium using a thermoplastic resin as the adhesive. A variety of sheet materials are useful as the facing layer. The useful materials may roughly be classified into woven or nonwoven cloths, soft plastics sheets, artificial leathers and laminated materials given by backing any of these sheet materials with a cushioning layer such as a urethane foam layer. Since walls (including ceiling) to be covered with these trimming boards usually have curved surfaces, the trim boards are produced as at least partly curved boards in conformance with the curved wall surfaces. Press-forming with application of heat is a usual way of producing curved trim boards of corrugate paperboard base. In most cases the substrate alone is formed into a desired shape by means of a hotpress, followed by the application of the facing layer onto the shaped substrate, but sometimes the facing layer is placed on a flat substrate with the interposal of a hot-melt type adhesive layer to accomplish the bonding simultaneously with the shaping of the substrate by heat-pressing.
However, press-forming of the corrugated paperboard substrate has encountered a problem, regardless of the presence or absence of the facing layer at the time of press-forming, originated from the fact that corrugated paperboard inherently lacks ductility. When the substrate is press-formed into a partly or entirely curved board, cracks tend to appear in curved areas particularly on one side which undergoes a larger extent of stretching during press-forming to give a convex surface. The probability of such cracking increases when the substrate is subjected to deep-drawing or made to bulge or dent with comparatively small radii of curvature in a small area where it is intended afterward to make either a bolt hole for fixing the trim board to wall or an aperture for the attachment of a certain instrument, for example a room lamp in the case of an automobile roof trim board.
Usually the aforementioned cracks can be hidden from view when the shaped substrate is laid with the facing layer, but, even though invisible, these cracks tend to grow larger after the application of the trim board to a wall or ceiling which is subject to externally caused vibrations and shocks. Other than the possibility of causing break of the substrate, the presence of cracks in the substrate frequently results in that the outer surface of the facing layer shows visible traces of the cracks since the facing layer is made to have only a small thickness, in most cases about a few millimeters, from the viewpoints of weight and cost. The presence of such traces of course impairs the decorative effect and hence the commertial value of the trim board.